Hormones are chemical messengers, produced by the adrenal, pituitary, thyroid, ovaries, testes and other glands, that have far-reaching effects throughout the body. Hormones regulate everything from growth and tissue repair to metabolism, reproduction, blood pressure and the body's response to stress. Hormones also regulate complex bodily functions, such as growth and sexual development.

The word hormone comes from the Greek word horman meaning "to set in motion." All multi-cellular organisms produce hormones, including plants. The system of glands that release hormones is referred to as the endocrine system. The branch of medicine that studies the endocrine system is known as endocrinology. The endocrine system is one of the body's main systems for communicating, controlling and coordinating the body's work.

The concept of internal secretion was developed in the 19th Century by Claude Bernard who described it in 1855, but did not specifically address the possibility of secretions of one organ acting as messengers to others. Still, various endocrine conditions were recognized and even treated effectively. The major breakthrough was the identification of secretin, the hormone secreted by the duodenum that stimulates pancreatic secretions, by Ernest Starling and William Bayliss, in 1902. Previously, it was thought that the process was regulated by the nervous system. Starling and Bayliss demonstrated that injecting duodenal extract into dogs rapidly increased pancreatic secretions, raising the possibility of a chemical messenger.

During the remainder of the 20th Century, all the major hormones were discovered as well as the cloning of the relevant genes and the identification of the many interlocking feedback mechanisms that characterize the endocrine system.

Hormones are commonly given as medications. Estrogens and progestagens are often prescribed as contraceptives to prevent pregnancy. Thyroxine is prescribed to individuals with an under or over-active thyroid. Individuals with autoimmune disorders and those undergoing gender reassignment procedures often take steroids. Diabetics take insulin to regulate the amount of sugar in their blood.

The endocrine system is one of the body's main systems for communicating, controlling and coordinating the body's work. It works with the nervous system, reproductive system, kidneys, gut, liver and fat to help maintain and control body energy levels, reproduction, growth and development and responses to surroundings, stress, and injury.

The endocrine system accomplishes these tasks through a network of glands and organs that produce, store and secrete certain hormones. Hormones cause an effect on other cells or tissues of the body.

Endocrine glands make hormones that are used inside the body. They produce and store hormones and release them as needed. When the body needs these substances, the bloodstream carries the hormones to specific targets. These targets may be organs, tissues or cells. Hormones usually are carried in the bloodstream.

Most cells are capable of producing one or more, sometimes many, molecules that signal other cells to alter their growth, function or metabolism. The classical endocrine glands and their hormone products are specialized to serve regulation on the overall organism level, but can often be used in other ways or only on the tissue level.

The rate of production of a hormone is often regulated by a homeostatic control system, generally by negative feedback. Homeostatic regulation of hormones depends, apart from production, on the metabolism and excretion of hormones. The internal balance of bodily systems is called homeostasis.

Hormone secretion can be stimulated and inhibited by: other hormones (stimulating or releasing hormones), plasma (the liquid portion of the blood that contains numerous proteins and minerals necessary for normal body functioning) concentrations of ions or nutrients, as well as binding globulins, neurons and mental activity and environmental changes (e.g. of light or temperature). A variety of illnesses, such as cancer or HIV/AIDS, may alter hormone levels in the body.

Steroid hormones: The adrenal cortex and the gonads are primary sources of steroid hormones, which are derived from cholesterol. Sterol hormones, including vitamin D derivatives such as calcitriol, are a homologous system. Steroid hormones may include: glucocorticoids (cortisol), mineralocorticoids (aldosterone). Sex steroid may include: androgens (testosterone, dehydroepiandrosterone or DHEA, dehyrdopiandrosterone sulfate or DHEAS, androstenedione and dihydrostesterone or DHT), estrogens (estradiol) and progestagens (progesterone and progestins).

Lipid and phospholipid hormones: This class of hormones is derived from lipids such as linolenic acid and phospholipids such as arachidonic acid. The main class is the eicosanoids, which includes the widely studied prostaglandins. Lipid hormones may include: prostaglandins, leukotrienes, prostacyclin and thromboxane.

Adrenal glands: The body has two adrenal glands, each above one of the two kidneys. Each adrenal gland is actually two endocrine organs. The outer portion is called the adrenal cortex. The inner portion is called the adrenal medulla. The hormones of the adrenal cortex are essential for vital life processes while the hormones of the adrenal medulla are not. The adrenal cortex produces glucocorticoids (such as cortisol) that help the body control blood sugar, increase the burning of protein and fat and respond to stressors like fever, major illness and injury. The mineralcorticoids (such as aldosterone) control blood volume. They also regulate blood pressure by acting on the kidneys to help them hold onto enough sodium and water. The adrenal cortex also produces some sex hormones, which are important for some secondary sex characteristics or the bodily changes that occur in women and men at puberty. The adrenal medulla produces epinephrine (adrenaline), which is secreted by nerve endings and increases the heart rate, opens airways to improve oxygen intake and increases blood flow to muscles, usually when a person is scared, excited or under stress. This is the hormone responsible for what is known as the "fight or flight" response. Norepinephrine also is made by the adrenal medulla, but this hormone is more related to maintaining normal activities rather than reacting to emergency situations.

Hypothalamus: The hypothalamus lies just above the pituitary gland at the base of the brain. It releases hormones that start and stop the release of pituitary hormones. The hypothalamus controls hormone production in the pituitary gland through several releasing hormones. Some of these are growth hormone-releasing hormone, or GHRH (controls GH release); thyrotropin-releasing hormone, or TRH (controls TSH release); and corticoptropin-releasing hormone, or CRH (controls ACTH release). Gonadotropin-releasing hormone (GnRH) tells the pituitary gland to make luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which are important for normal puberty.

Ovaries (women): The ovaries are located at about the level of the pelvis, on opposite sides of the uterus. The two most important hormones of the ovaries are estrogen and progesterone. These hormones are responsible for developing and maintaining female sexual traits, as well as maintaining pregnancy. Along with the pituitary gonadotropins (FH and LSH), they control the menstrual cycle. The ovaries also produce inhibin, a protein that hinders the release of follicle-stimulating hormone from the anterior pituitary and helps control egg development. The most common change in the ovarian hormones is caused by the start of menopause, part of the normal aging process. It also may occur when ovaries are removed surgically. Loss of ovarian function means loss of estrogen, which can lead to hot flashes, thinning vaginal tissue, lack of menstrual periods, mood changes and bone loss or osteoporosis.

Pancreas: The pancreas is a large gland behind the stomach that helps the body to maintain healthy blood sugar (glucose) levels. The pancreas secretes insulin, a hormone that helps glucose move from the blood into the cells where it is used for energy. The pancreas also secretes glucagon when the blood sugar is low to tell the liver when to release glucose into the bloodstream.

Parathyroid: Located behind the thyroid gland are four tiny parathyroid glands. These make hormones that help control calcium and phosphorous levels in the body necessary for proper bone development. In response to too little calcium in the diet, the parathyroid glands make parathyroid hormone, or PTH, that takes calcium from bones so that it will be available in the blood for nerve conduction and muscle contraction.

Pineal gland: There is little scientific knowledge of the pineal gland. Thus far, one hormone produced by this gland has been found, melatonin. Melatonin may stop the action of the hormones that produce gonadotropin, which causes the ovaries and testes to develop and function. It may also help to control sleep patterns.

Pituitary gland: The pituitary gland's function is complex and important for overall well-being. The pituitary gland is divided into two parts: front (anterior) and back (posterior). The anterior pituitary produces several hormones. Prolactin or PRL stimulates milk production from a woman's breasts after childbirth and may affect sex hormone levels from the ovaries in women and the testes in men. Growth hormone or GH stimulates growth in childhood and is important for maintaining a healthy body composition. In adults it is also important for maintaining muscle mass and bone mass. It may affect fat distribution in the body. Adrenocorticotropin or ACTH stimulates production of cortisol by the adrenal glands. Cortisol, a stress hormone, is vital to survival because it helps maintain blood pressure and glucose levels. Thyroid-stimulating hormone or TSH stimulates the thyroid gland to make thyroid hormones, which, in turn, regulate the body's metabolism, energy, growth and development and nervous system activity. Luteinizing hormone or LH controls testosterone in men and estrogen in women. Follicle-stimulating hormone or FSH promotes sperm production in men and stimulates the ovaries to release eggs (ovulate) in women. LH and FSH work together to allow normal function of the ovaries or testes. The posterior pituitary produces two hormones. Oxytocin causes milk letdown in nursing mothers and contractions during childbirth. Antidiuretic hormone or ADH, also called vasopressin, is stored in the back part of the pituitary gland and regulates water balance. If this hormone is not secreted properly, it may lead to problems of salt and water balance and may harm kidney function. In response to the over or underproduction of pituitary hormones, the target glands affected by these hormones may produce too many or too few hormones of their own.

Testes (men): The testes, twin reproductive glands that produce the hormone testosterone, are located near the penis. Testosterone is the primary hormone responsible for the development and maintenance of a male's sexual traits. During puberty, testosterone helps to bring about the physical changes that turn a boy into an adult male, such as growth of the penis and testes, growth of facial and pubic hair, deepening of the voice, increase in muscle mass and strength and increase in height. Throughout adult life, testosterone helps maintain sex drive, sperm production, male hair patterns, muscle mass and bone mass.

Thymus: Located on the upper chest, about level with the breastbone, the thymus is an organ that is large at birth and necessary for normal immune function. The thymus weighs its greatest at puberty. In adolescence, its tissue is replaced by fat. The thymus gland secretes hormones called humoral factors. These hormones help to develop the lymphoid system, which is a system throughout the body that helps it to reach a mature immune response in cells to protect them from invading bodies. Thymus hormones play a role in regulating the immune system by stimulating other kinds of immune cells.

Thyroid: A small gland inside the neck, located in front of the trachea (breathing airway) and below the Adam's apple. Thyroid hormones control metabolism, which is the body's ability to break down food and store it as energy and the ability to break down food into waste products with a release of energy in the process. The thyroid produces two hormones, T3 (tri-iodothyronine) and T4 (thyroxine); they control the speed at which the body's chemical functions proceed (metabolic rate). Thyroid hormones influence the metabolic rate in two ways: by stimulating almost every tissue in the body to produce proteins and by increasing the amount of oxygen that cells use. Thyroid hormones affect many vital body functions including: heart rate, respiratory rate, rate at which calories are burned (metabolism), skin maintenance, growth, heat production, fertility and digestion.

The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.